Because this adds a new class template called std::unexpected, we have
to stop declaring the std::unexpected() function (which was deprecated
in C++11 and removed in C++17).
libstdc++-v3/ChangeLog:
* doc/doxygen/user.cfg.in: Add new header.
* include/Makefile.am: Likewise.
* include/Makefile.in: Regenerate.
* include/precompiled/stdc++.h: Add new header.
* include/std/version (__cpp_lib_expected): Define.
* libsupc++/exception [__cplusplus > 202002] (unexpected)
(unexpected_handler, set_unexpected): Do not declare for C++23.
* include/std/expected: New file.
* testsuite/20_util/expected/assign.cc: New test.
* testsuite/20_util/expected/cons.cc: New test.
* testsuite/20_util/expected/illformed_neg.cc: New test.
* testsuite/20_util/expected/observers.cc: New test.
* testsuite/20_util/expected/requirements.cc: New test.
* testsuite/20_util/expected/swap.cc: New test.
* testsuite/20_util/expected/synopsis.cc: New test.
* testsuite/20_util/expected/unexpected.cc: New test.
* testsuite/20_util/expected/version.cc: New test.
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